1. Field of the Invention
The present invention relates to an image correction apparatus for correcting images which are displayed in a color television receiver, and more particularly to an image correction apparatus for automatically making various corrections such as convergence correction, geometric distortion correction, brightness correction, and focus correction.
2. Description of the Related Art
Generally, in a projection type display or video projector for projecting enlarged images onto a screen by using three projection tubes respectively producing three primary colors, conditions such as color separation, brightness variation and out-of-focus may occur in displaying the images on the screen. This is because the incident angles of the respective projecting tubes with respect to the screen are different from one another.
These conditions can be corrected manually by viewing images with the eye. However, such a work requires a large amount of time for correcting images.
A digital convergence apparatus, implementing a method for achieving highly accurate convergence, is disclosed in Japanese Patent Publication No. 59-8114. An automatic convergence correction apparatus, implementing a method for automatically correcting deflection distortion, is disclosed, for example, in Japanese Patent Publication Nos. 3-38797 and 1-48553, and U.S. Pat. No. 4,999,703. A convergence error correcting method for detecting and correcting convergence errors is disclosed in Japanese Laid-open Patent Publication No. 64-54993. A convergence error correction apparatus for a projection type display, implementing a method for automatically detecting and correcting convergence errors, is disclosed in Japanese Laid-open Patent Publication No. 63-48987.
FIG. 38 shows a configuration of a prior art automatic convergence correction apparatus for automatically correcting misconvergence.
The automatic convergence correction apparatus includes a display device 101 which is to be adjusted for convergence; a signal generator 102 for generating a signal for adjusting the convergence; a signal selector 103; an imaging device 104 for capturing an image displayed on the display device 101; an image processor 105 for calculating the centroid and detecting the amount of misconvergence; and a controller 106 for controlling the signal generator 102, the signal selector 103 and the image processor 105.
The operation of the automatic convergence correction apparatus having the configuration mentioned above will be described below.
The signal generator 102 generates a repetitive pattern of a low frequency which has the waveform shown in FIG. 39. In FIG. 39, the x-axis corresponds to the horizontal direction of the display screen and y-axis corresponds to the vertical direction of the display screen. This repetitive pattern is supplied to the display device 101 through the signal selector 103. The display device 101 displays the repetitive pattern.
The imaging device 104 captures the repetitive pattern displayed on the display device 101 and supplies an image signal to the image processor 105. The image signal has a waveform including at least one crest portion at which the amplitude of the image signal takes a local maximum value. Hereinafter, the crest portion of the image signal is referred to as a centroid of the image signal.
The image processor 105 calculates a position of the centroid of the image signal for each of three primary colors R(red), G(green) and B(blue). The difference between the respective positions of the centroids for the primary colors is used to calculate the amount of misconvergence.
The calculation of the position of the centroid will be described below in detail. First, the image signal representing the repetitive pattern output from the imaging device 104 is A/D converted, and the resulting digital data is then linearly interpolated.
In FIG. 40, the solid curve hi(x) shows a part of the resulting image signal representing the repetitive pattern and the broken curve shows a second-order curve which is approximate to the curve hi(x). The remaining part of the resulting image signal is similar to the part shown in FIG. 40 and therefore, the description thereof is omitted.
The position of the centroid is calculated according to the following equation by the use of the second-order curve approximation method. ##EQU1##
The integral range L.sub.i of the equation depends on a threshold value hTH. Here, A.x.sup.2 +B.x+C represents a second-order curve, and the coefficients A, B and C are determined so that the value D of the equation becomes the smallest.
More specifically, the coefficients A, B and C are determined so that the conditions of .differential.D/.differential.A=0, .differential.D/.differential.B=0 and .differential.D/.differential.C=0. Then, the position of the centroid x0 is given by x0=-(B/2A).
The above-described second-order curve approximation method is repeatedly applied for each of the primary colors R, G and B so as to calculate the respective positions of the centroids. The automatic convergence correction is achieved by calculating the difference between the respective positions of the centroids as the amount of misconvergence and then adjusting the amount of misconvergence.
However, the above prior art automatic convergence correction apparatus has a problem in terms of operating speeds. This is because the image processor is required to perform complex operations since the second-order curve approximation method is employed to calculate the positions of the centroids of the repetitive patterns for adjusting convergence.
Furthermore, in the above mentioned prior art automatic convergence correction apparatus, an image signal having an undulating symmetric waveform is output from the imaging device 104. Based on the image signal, the image processor 105 calculates the position of the centroid by the use of the second-order curve approximation method. This presents a problem in that the accuracy of the centroid calculation is reduced when the image signal output from the imaging device 104 does not have an undulating symmetric waveform for whatever reason such as shading in the video projector and the gamma characteristics of the display device.
Moreover, in the prior art automatic convergence correction apparatus, the scanning line direction of the imaging device 104 is used as the reference in calculating the position of the centroid. No consideration is given to a positional relationship between the display device 101 and the imaging device 104. As a result, the apparatus has had the problem that when the imaging device 104 is tilted with respect to the horizontal scanning direction of the display device 101, the displayed image is tilted even if correction is made.